'antidisestablishmentarian' is formed from the prefix 'anti-' (from Greek 'anti' meaning 'against'), the element 'dis-' (from Latin 'dis-' meaning 'apart' or 'reversal'), the word 'establish' (from Old French/Latin roots meaning 'to make firm or set up'), and the suffix '-arian' (from Latin '-arius', forming nouns meaning 'person associated with').

'antidisestablishmentarian' emerged in 19th-century English political discourse describing those opposed to proposals to remove the Church of England's status as the established church; it combines 'antidisestablishment' + the agentive suffix '-arian' to mean 'a person who holds that position.'

Initially it specifically meant 'a person opposed to the disestablishment of the Church of England'; over time it has also come to be used more broadly for opponents of disestablishment in other contexts and, colloquially, as an example of a very long word.